The production of multiple layer optical material structures has become very advanced over the years, and requires careful control during the processing stage to ensure that the layers being deposited, or grown, are of the desired thickness. Additionally, supervision of the actual refractive index, or a function thereof, for each layer, would be a useful metric of the appropriate composition of the layer. As layers are added, one on top of the other, care must be taken to ensure that a clean boundary between layers is defined, and that the boundary does not suffer from diffusion as additional layers are added.
The prior art teaches cleaving a section of the structure, and then analyzing the cleaved section in a scanning electron microscope. Unfortunately this testing suffers from 2 drawbacks, namely it is destructive and slow. To overcome some of these difficulties, Fourier Transform Infrared Spectroscopy was developed, wherein a sample is irradiated with infrared light having a relatively wide wave number range, followed by Fourier transformation of the resultant interference spectrum to produce a space interference waveform. Unfortunately, a direct result of the desired properties and metrics indicated above are not available from the space interference waveform according to the prior art, and instead a numerically intensive method of utilizing an optical characteristic matrix is described, such as in U.S. Pat. No. 5,587,792 issued Dec. 24, 1996 to Nishizawa et al., the entire contents of which is incorporated herein by reference. Such a numerically intensive method causes in-situ evaluation to be cumbersome and relatively slow, in particular as interpretation of the results for a non-trivial number of layers is not direct, but is instead based on curve fitting against theoretical models.
A bilinear transformation of reflectance has been proposed for analysis of the optical thickness. Specifically, a bilinear transformation of reflectance data is followed by a Fourier transform and hence transformed to the optical thickness domain, and the optical thickness peaks thus provide an analysis of the optical thickness of the actual structure. Unfortunately, such a method yields direct results only for small refractive index steps, i.e. wherein the structure to be analyzed does not exhibit refractive index steps greater than about 20%. In the event of large refractive index steps, such a transformation yields numerous peaks in the optical thickness domain, the number of peaks exceeding the number of interfaces. Thus, this method has been deemed unsuitable for analysis of multiple layer optical material structures with large refractive index steps.